Projection displays are used for a wide variety of applications, such as producing the pictures viewed on television screens. A typical projection display system includes a number of components, including a display screen, a light source, and an optical path between them. To create the pictures, one or more light sources are provided to emit light when it is needed. The light they produce is then manipulated by a series of optical devices in order to create the visual image. The visual image created along the optical path is then displayed on the display screen, the television screen for example, or another visual display. In most cases, the goal is to produce the best picture possible. The key to producing a desirable visual display, of course, is the configuration of the various optical devices along the optical path. The selection, operation, and configuration of these devices also contribute to unseen characteristics of the system, such as cost, size, and efficient use of system resources.
Several types of projection displays have recently been developed. These new display systems are now becoming much more common, serving as a replacement for the widely-used CRT (cathode ray tube) display, which produces a visual image by producing and directing a stream of electrons at a treated display surface. The stream could only be directed to one point at any given time, but can be systematically swept across the display with such speed as to create the visual impression of a single image. This technology is fairly well-developed, but has reached the point where perceptible increases in quality are difficult to achieve. A CRT also takes up a relatively-large amount of space because the components used for generating the electron stream must be placed at a certain distance from the display screen. Many recently-developed projection display systems, in contrast, feature a much slimmer profile. In addition, projection display systems often produce much cleaner visual images. The combination of these advantages has made such systems immensely popular.
One such projection-display system is a spatial light modulator (SLM) commercially available from Texas Instruments of Dallas, Tex. under the trademark DLP® (or Digital Light Processing®). DLP® projection-display systems utilize a digital micromirror device (DMD) in their optical path. The DMD typically includes an array of thousands of tiny mirrors that are used to manipulate colored light originating at an internal light source. Lenses and other components in the optical path adjust the light for use by the SLM, or convey the image it generates to a display plane. The colored light is reflected by the SLM and projected onto a display plane for viewing according to an input image. Projection lenses may be used to magnify and/or focus the image on the display plane.
It should be noted that the light source may be any type of light source, including a light-emitting diode (LED), a lamp, a laser, or the like. Some light sources, particularly LEDs, emit light that exhibit Lambertian characteristics. As illustrated in FIG. 1, an LED 100 typically emits light over a wide solid angle, i.e., the light is not directed or collimated along a specific direction. As a result, display systems typically include one or more optical elements to image the LED source light onto the DMD without degrading the etendue at the DMD. The concept of “etendue” refers to the geometric extent of a source or any other component having a limited area and acceptance cone angle. In the context of an SLM-based system, the source of light incident on the SLM has an etendue that corresponds to the size and directionality of the source, and the SLM also has an etendue that corresponds to its size and ability to receive light from various directions. It is desirable that the etendue of the LED and the SLM bematched, or in other words, the high divergence with a small area of the LED is equal to the lower divergence with a bigger area of a SLM such as a DMD. The main challenge is to image the LED source by reducing the light source emission angle onto the DMD without degrading that same etendue (if too much aberration is present making the image blurred and less “bright” than the source).
These attempts, however, typically require a series of lenses that must be in strict alignment relative to each other as well as the LED light source and the SLM. Due to the alignment issues, the lens system can be extremely difficult and expensive to produce.
Accordingly, there is a need for a system and method to collimate light that is easy and efficient to produce.